Pump



H. E. ROSE Feb. 16, 1960 PUMP Filed June 6, 1957 frzuerzZEr: fi awczrd Z .5056 @"fia/ml United States Patent PUMP Howard E. Rose, Hudson, Ohio, assignor to Borg-Warner Corporation, Chicago, Ill., a corporation of Illinois This invention relates to pumps in which a plurality of pump cylinders are arranged in circular series parallel to each other and to the axis of the drive shaft, each of the cylinders having disposed therein a piston or a plunger actuated by a cam or swash plate fixed on the shaft.

The pump forming the subject matter of this invention is provided with means to vary the displacement thereof, such means including means responsive to the discharge pressure of the fluid from the cylinders, and the pump is further provided with means allowing a selection of a particular maximum discharge pressure therefrom. Such means are desirable in pumps which are constantly running, as for example, those used in aircraft installations and where the displacement requirements vary from a maximum to a zero value. The structure forming the subject matter of the instant application includes a variance of the means for accomplishing the desired results described and claimed in copending applications of Francis E. Norlin, Serial No. 573,543, filed March 23, 1956, entitled Variable Displacement Hydraulic Pump, and Serial No. 598,718, filed July 18, 1956, entitled Pump.

Therefore, the primary object of this invention is the provision of an improved variable displacement pump of the type incorporating a plurality of circularly arranged cylinders having pistons therein actuated by means of a cam or swash plate.

More particularly, the subject pump includes pistons, each having an open-ended cavity therein in which is disposed a valve means adapted to close the open end of the piston cavity together with means for varying the position of closing the open end of the piston cavity by means of the valve during the piston discharge stroke to thereby eifectively vary the effective discharge stroke of the piston and the displacement of the pump, the latter means including a pilot valve responsive to pump discharge pressure. The piston valve means of the subject pump may include a poppet valve or a disc valve, each mounted for limited axial movement within the piston. The means for varying the effective discharge stroke of the pistons incorporates a substantially annular plate mounted for limited axial movement which cooperates with the aforementioned valve means in a novel manner and by its movement changes the relative closing position of the valve means with regard to the piston. Upon movement of the plate in one direction, resilient means in the piston cavity cause the valve means to remain open during a greater portion of the piston discharge stroke, and upon movement of the plate in the other direction, the resilient means are opposed in their action on the valve means. The annular plate is moved in one direction by means of an associated fluid motor having a piston responsive to a pressure related to the pump discharge pressure and returned in the opposite direction by spring means opposing the movement thereof. The limits of movement of the plate are such to correspond to full and zero pump displacements.

Still another object of this invention is the provision of an improved swash plate pump having a plurality of pistons reciprocated within cylinders so as to each have a suction stroke and a discharge stroke, valve means associated with each of the pistons, and an axially movable annular plate cooperating with the piston structure to vary the relative closing position of the piston with regard to its cylinder, thereby the displacement. of the pump.

Another and still further object of this invention is the provision of an improved swash plate variable displacement pump including an annular plate responsive to a fluid pressure related to the discharge pressure of the pump to vary the displacement thereof.

These and other objects and features of this invention will be apparent from the following specification when taken with the accompanying drawing, in which:

Figure l is a partial cross-sectional view of one embodiment of this invention and illustrates to advantage the piston valve means thereof;

Figure 2 is a partial cross-sectional view of another embodiment of this invention and illustrates to advantage the piston valve means thereof; and

Figure 3 is an end view of the piston and valve illustrated in Figure 2.

Figure 4 is a partial cross-sectional view illustrating a pump assembly formed in accordance with the teachings of this invention.

Referring now to the drawing, there is illustrated a pump, generally indicated at 10, including a housing A .having an inlet connection B and an outlet connection C in the same configuration as disclosed in the aforementioned copending applications.

A cylinder assembly composed of a cylinder barrel block 11 and a guide block 12 is clamped within the housing A. The blocks 11 and 12 are annular in form and define with the housing A an inlet chamber 13 which communicates with the inlet port by way of a suitable passage, and cylinder barrel block 11 defines with the housing a discharge chamber 14 which communicates with the outlet port by way of a suitable passage.

The cylinder block 11 is bored through to form a plurality of radially arranged cylinders 15, and the block 12 is formed to define with the block 11 inlet passages 16 which communicate with the cylinders 15. The usual number of cylinders in a pump of this type is nine (9). The guide block 12 is bored through, as at 17, the guide block bores being of the same diameter and in alignment with the cylinders 15. Each cylinder 15 has a discharge valve D associated therewith of the same type as shown and described in the aforementioned copending applications, seating on the outer face of the cylinder barrel block 15 and the valves are disposed with the discharge chamber 14.

A piston or plunger 18 is slidably received in each of the cylinders 15 and extends through the guide block bores 17 and the block member 12, terminating in an enlarged shoulder portion 19 located in a cavity 20 within the housing. Each piston 18 is provided with a cavity 21.therein open at the end thereof adjacent the associated discharge valve, and an axially elongated inlet port 22 in communication with the inlet passage 16.

A suitably journalled swash plate or cam E disposed within the cavity 20 is provided for reciprocating the pistons or plungers 18 between their full suction and full discharge positions. The swash plate or cam E is rotated by a drive shaft F suitably connected to the plate or cam B.

Each piston 18 is provided with a substantially hemispherical cavity 23 in its enlarged portion 19 to receive a complementary-shaped bearing shoe 24 having a flat surface which bears against the cam or swash plate.

Patented Feb. 16, 19so 3 'Ihe'p'istons '18 are retracted'by means of a piston return "plate having "forked portions *engageable with the enlarged portions of the pistons, and the return plate 25is guided for wobbling motion by means of an end portion '26 of a pilot valve retainer 27 received'in a central 'bore '28 in the guide block 11.

A valve means, generally identified as 29,'is provided for opening and closing the open end of each piston during the stroke thereof. As is obvious from the drawing, two embodiments of the valvemeans are illustrated. In Figure 1, a poppet type is shown, while in Figures .2 and 3, a disc type is shown. The valve means 29, Figure 1, is of the poppet type and comprises a valve proper 30'adapted to seat'on the open end of the piston 18 "having an axially extending stem 31 guided for axial movement within the piston 18 by means of spaced guides 32 having relieved sides for the passage of fluid. Acoiled compression spring 33 surrounds the stem 31 andis disposed between the guides 32 to resiliently urge and maintain them in their-spaced relation. A threaded nut 34 is-received on the stem 31 and is restrained from movement by means of a'pin 35 extending therethrough and through the stem. As will be obvious,-the nut, by engagement with the right guide 32 limits the axial move- "ment of the valve stem and the valve proper 30.

A spring retainer 36 is slidably received in the piston cavity '21 and is provided with suitable openingsfor the passage of fluid. The retainer 36 is resiliently urged toward the valve 29 by means of a coiled compression spring 37, also received within the cavity '21.

The valve means 29, Figures2 and 3, is of the disc type and comprises a disc or flapper valve proper 38 mounted for .limited axial movement within opposed 'flanged, extension portions 39 of the piston 18, so thatwhen the disc is in its Figure -2 position, the cavity 21 will be closed and when moved to the left, from its illustrated position, the cavity 21 will be open. As will be noted, the disc or flapper 38 is provided with side ex tensions 40, 'so as to contact thepiston 18 to properly position the valve.

:A spring retainer 41 provided with suitable openings for the passage of fluid is received in the piston cavity 21 and isurged toward the disc or flapper 38 by means of a coiled compression spring 42, also received within the 'cavity 21.

The'retainer 41is 'constructed'with an axially extending stem 43'terminati'nign an enlarged end 44 adjacent the valve proper 38, the enlarged end being cutaway ('as'indicated in Figure 3) for the passage of fluid.

By varying the position, relative to the cylinder 15, in which the valve'means 29 closes the open end of the piston '18, the effective discharge stroke of the piston will be varied, and such variation will thereby vary the displacement of the pump.

Inorderto vary the position at which the valve means 'closes the inlet passage 16, a substantially annular plate 45 is provided which extends through the elongated port 22 in'each piston 18 to a position where it engages the .retainer36 in Figure 1, or the retainer 41 in Figure 3. .The annular plate-45 is mounted on an annular support 46 and the plate and support are axially movable between certain limits corresponding to zero and full ,pump displacement. The plate 45 and support 46 are supported for such axial movement and travel with a cylindrical extension 47 of an annular plate Piston 48 disposed Within a cavity 49 in the guide block 12, the piston 48 defining with the cavity 49 a chamber 50 which will :be referred to as the platepiston chamber.

The annular plate 45 is resiliently urged towards the left by means of coiled compression springs 51 received in suitable cavities in the plate piston and acting against the support -46. .A cylindrical, flanged, retainer 52 is received around the cylindrical extension 47 of the plate piston and abuts the plate 45, being resiliently urged thereagainst xme nst :a plateandrpi tonreturn spring 4 53 supported at its opposite end by a portion of the housing, so that upon movement of the piston 48 and the plate 45, the spring 53 will be compressed. The axial movement of the plate 45 is limited in one direction by a stop surface 54 formed on the cylinder barrel block 11 and by the piston 48 in the other direction.

The pilot valve retainer 27, received in the bore 28, includes a portion received within the plate piston 50. The retainer 27 is provided with open-ended concentric communicating bores 55 and 56 and is drilled as at 57 to form a communication between the outer periphery thereof and the bore 56. The retainer 27 is also provided with a reduced groove portion 58 defining with'the plate piston 48 and the bore 28 a chamber 59 for a later to be described purpose.

The chamber 59 is in communication with the plate piston chamber 50, as illustrated. The retainer 27 is also provided with bores or openings 60 and 61 providing communication between the bore 55 and the chamber .59.

A'spool-type pilot valve 62 is slidably received within the bore '55 and is provided with an open-ended cavity 63, a first reduced portion 64, a groove 65, a second reduced portion 66 and a third reduced portion 67, the latter reduced portions defining a shoulder 68 at their juncture. bore '55 surrounding the third reduced portion 67 of'the pilot valve 62 and is fixedly retained in position by means of a fastening element 70. The sliding movement of the pilot valve 62 is limited by the shoulder 68 engaging the sleeve 69 in one direction, and the end of the pilot valve '62 engaging the shoulder formed by the intersection of the bores 55 and 56 in the other direction. The pilot valve '62 is bored, as at 71, to provide communication between the 'groove 65 and the bore 63, and thesec- 0nd reduced portion 66 together with a part of the third reduced portion 67 and the pilot valve'sleeve '69 define with the bore 55 a'pilotring pressure chamber 72.

A pin 73'is disposed within the bore 63 at one end and within a cavity in a cup-like spring retainer 74 at its other end. The spring retainer 74 is axially elongated and has annular portions slidably engageable with the interior of the cylindrical extension 47 of the plate piston 48. A coiled compression spring 75, hereinafter referred to as the pilot valve spring, is disposed within I the spring retainer 74 at one end and at its other end is supportedby'means of the pump housing. The loading of the spring 75 may bevaried as described in the aforementioned copending applications. The spring 75 resiliently urges the pilot valve 62 to the right through the pin 73. Openings 76 are provided in the retainer 74, and openings are also provided in the extension for the passage of fluid therethrough, the'latter openings not being shown. a

The guide block 12 is provided with an annular chamber 77 in communication with the bores or passages '57, and is also provided with an annularly disposed passage 78 communicating at one end with the cavity 77 and at the other end with the dicharge chamber 14 for the passage of discharge fluid to the cavity 77.

As in pumps of the type, suitable O-ring sealing means may be provided where necessary or desirable.

The following description sets forth the operation'of the two embodiments of'the pump and this operation'will be described with reference to one of the plurality of pistons of each, it being understood that the operation of all pistons in each pump is. the same.

Assuming the cam or swash plate E to be rotating, the pistons 18 will be recipro'cated within their cylinders 15 between their suction positions and their discharge positions. Fluid from a suitable source will flow to the inlet, .into the inlet passage, into the chamber .13, filling the entire chamber, from the chamber 13 through the passage 1 and wh the p on 18 is in its suctionP A pilot valve sleeve 69 is disposed withinthe 'tion, through the inlet opening 22 into the cavity 21 of the piston 18. Since the piston valve means 29 is supported, in each embodiment for free, limited, axial moveinent, the fluid within the piston cavity 18 will cause the valve proper 30 or 38, as the case may be, to move away from the end of the piston, so that fluid will flow around the valve proper into the cylinder 15 in advance of the piston. On the discharge stroke of the piston 18 the fluid within the cylinder 15 will be compressed causing the valve proper to close.

As the piston 18 moves to the left in its discharge stroke, compressing the fluid trapped within the cylinder 15, the discharge valve associated therewith will be unseated, so that the fluid under pressure will flow to the outlet. At the same time, high pressure fluid will flow through the passage 78 and into the chamber 77. The fluid in the chamber will flow through the openings and into the bore where it contacts one end of the spool-type pilot valve.

When the pressure in the bore 56 becomes sufliciently high, or, in other words, the discharge pressure becomes sufliciently high, so that the force on the end of the pilot valve 62 overbalances the spring 75, the pilot valve 62 mo'ves to the left, as illustrated in the drawing, and compresses the pilot valve spring 75. The pilot valve spring 75 may be adjusted, as before mentioned, to a load which will allow the pilot valve 62 to reach a position providing a bleed between the reduced portion 64 and the opening 61 at a predetermined outlet pressure. A slight increase in outlet pressure will allow the flow o'f fluid in the reduced portion 64 to bleed into the plate piston chamber 50 through the opening 61. At this time it will be noted that the groove 58 is blocked by the interior of the bore 55. Pressure will build up in the plate piston chamber 50 to a valve sufficient to overcome the plate return spring 53, and the fluid thus acting on the piston 48 will start moving the plate 45 to the left, as illustrated in the drawing.

Movement of the plate 45 will be accompanied by corresponding movement of the retainer 36 (Figure 1) or the retainer 41 (Figure 2) through the action of the springs 37 and 42, respectively. The retainer 36 and 41 will then contact the valve stem 31 or the valve proper 38, as the case may be, and cause the valves proper 30 and 38, respectively, to remain open for a longer period of time during the piston discharge stroke, thereby allowing more fluid in the cylinder 15 to bypass back to inlet and preventing the pressure in the piston cylinder 15 from building up until the piston 18 has reached a point in its travel toward its discharge position where the valve pro'per (either 39 or 38) closes the end thereof.

Further movement of the plate 45 increases the time of the valve proper remaining open, so that when the plate 45 has moved to its limit to the left, the valve proper will remain open for substantially the entire piston discharge stroke, and therefore, the displacement of the pump will be zero.

A slight decrease in outlet or discharge pressure will allo'w the pilot valve 62 to move toward its position illustrated and thereby shut ofl' the outlet flow to the chamber 50, and will allow fluid trapped in the chamber 50 to bleed out through the passage 61, into the groove 65, through the passage 71, through the bore 63 and to the .chamber 13 through the aforementioned openings 76 in the retainer 74 and the openings in the cylindrical extention 47. The plate return spring 53 will return the plate 45 to its illustrated position, which will reverse the action of the piston valve 29, increase the effective piston stroke, and thereby increase the displacement of the pump.

A steady, partial flow demand will cause the pilot valve 62 to assume a position in which pilot valve leakage or bleed into the piston chamber 50 will equal pilo't valve leakage out of the piston chamber 50 back to the inlet cavity 13, as earlier described, thus holding the plate 45 in a constant partial flow position. Since the pilot valve 62 travels only a few thousandths of an inch in the bore 55 to bleed flow into and out of the plate chamber 50, the increased load of the pilot valve spring 75 due to this travel is very slight. Also, since the pilot valve 62 assumes the same position for any partial flow demand, the load on the spring 75 is the same. and therefore, the outlet pressure will be the same. Thus, there would be no pressure differential between maximum pressure at full flow and at zero flow. But to control the location of the plate 45, there must be a pressure change and this pressure change, called the pressure diflerential, has been built into the pilot valve. Thus, a cut-off slope or angle is provided on a pressure versus flow curve, which corresponds to the pressure diflerential.

To provide the pressure diflerential is the function of the pilot ring pressure chamber 72. As will be obvious, when fluid flows to the chamber 50, there will also be a flow to the chamber 72, through the opening 60. When the chamber 72 is under pressure, it balances out part of the outlet pressure which contacts the end of the pilot valve 62. The pressure in the chamber 72 increases as the plate 45 mo'ves towards zero displacement position (the limit of the plate movement to the left, as illustrated) due to the increasing load on the plate return spring 53. As the pressure increases in the pilot ring pressure chamber 72, the outlet pressure of the pump must increase to overcome its pressure and this creates the pressure differential between full and zero flow.

Thus, the pilot ring pressure chamber 72 hydraulically applies the spring rate of the plate return spring 53 to the pilot valve 62. During partial displacements of the pump, the load of the plate return spring 53 varies with the position of the plate 45. As this load varies the pressure in the plate piston chamber 50 and in the pilot ring chamber 72 varies. Since the pilot ring chamber 72 applies this pressure to the pilot valve 62 against outlet or discharge pressure, the outlet pressure must vary to maintain the pilot valve 62 in balance. Therefore, a definite link has been established between the location of the plate 45 and outlet pressure, and since the location of the plate 45 establishes the correct displacement, it can be said that outlet pressure establishes the correct displacement.

Without this pilot ring chamber 72, the valve would crack at a set pressure and close off at a set pressure, regardless of the location of the plate 45, since the plate 45 can move from its full displacement position (as illustrated) to its zero displacement position (the limit of its movement to the left) without increasing the load of the pilot valve spring 75. Therefore, there would be no change in outlet pressure whether the pump be in, for example, 75% displacement, 50% displacement or zero displacement. If there were no change in outlet pressure there would be no means of stopping the plate 45 at any desired location, and as a result, the plate 45 would hunt back and forth, resulting in undesirable chatter.

While this invention has been described in connection with certain specific embodiments thereof it is to be understood that this is by way of illustration and not by way of limitation and the scope of this invention is defined solely by the appended claims which should be. construed as broadly as the prior art will permit.

I claim:

1. In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series of cylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an open-ended cavity in each of said pistons, valve'means carried by each piston and eflective to open and close said cavity; a movable member-,carried'by :each piston and adapted to engagesaidvalve means to vary the position relative to the cylinder at which said valve means closes said piston cavity, saidmovablemembers thereby being effective to vary the eflective discharge stroke of said pistons and the displacement of said pump, biasing means engaging said movable members and tending to urge said members to positions providing minimum displacement, a source of fluid pressure, and an annular plate mounted for limited axial movement in response to the pressure value ofsaid fluid pressure and engaging said members to determine the positions thereof relative to said valve means, said plate upon movement in one direction in response to an increase in the pressure value of said fluid pressure permitting said members to cause movement of said valve means :relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pump displacement.

,2. In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the'combination of a housing provided with an inlet and an outlet;.means defining a circular series of cylinder .bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein soas to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an open-ended cavity in each of said pistons, valve means carried'by each piston and effective to open and close said cavity; a movable member carried by each piston and adapted to engage said valve means to vary the position relative to the cylinder at which said valve means closes said piston cavity, said movabie members thereby being eifective to vary the effective discharge stroke ofsaid pistons and the displacement of said pump, a source of fluid pressnre, a fluid motor carried by said housing and responsiveito the pressure value of said fluid pressure, andtan annularplate interconnected with said fluid motor and mounted for limited axial movement'inrresponseto the pressure value of said fluid pressure, said plate upon movement in one direction in responseto an increase in the pressure value of said fluid pressure premitting said members to cause movement of said valve means relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upontmovement of said plate in another direction in response to .a decrease in the pressure value of said "fluid pressure to thereby increase pump displacement.

3.,A pump 'as recited in claim 2 wherein said fluid motorvin response to an increase in the pressure value of said fluid pressure above a predetermined pressure value causesmovernent of said plate in said one direction to thereby decrease pump displacement.

4.]A pump as recited in claim 3 wherein spring means engages said plate and opposes movement of said plate in Said one direction, said spring means being adapted to cause movement of said plate in said another direction when said fluid motor is ineffective to thereby increase pump displacement.

' .5.In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series ofvcylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons,- each. received in a cylinder bore for reciprocation thereinso as to having a suction stroke and a discharge stroke;arrotaryrdriveshaft; ,arcam carried by said drive shaft for reciprocating, said pistons; .an open-ended cavity inUeach roflsaid pistons, valve means comprisinga poppet having a valvegproper radapted .to 5333.: .on the ,gpen' end ofreach piston and eifective to open and ;close;saidl cavity; a movable member :carried rbyteachpiston and ,adapted to engage said valve means to .vary :the position relative .to the cylinder .at which saidvalve means closes said piston cavity, said movablernembers thereby beingietfective -to vary the eflective :discharge'strokeiof saidpistons and the displacement .of said pump, a source of fluid pressure, and an annular plate mounted for limi-tediax ial move ment in response to the pressure valveof saidfluid pressnre,:said plate upon ,movement in one direction in response to an increase in the-pressure vvalue of said fluid pressure permitting said members-to cause'movementof said valve means relative to said pistons whereby said cavities remain open during a greater portion of :the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to ,a decrease-in the-pressure value of said fluid pressure to thereby increase pump displacement.

6.31:1 a'pump of the type in which a plurality of cylinders are arranged in'circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series ,ofcylinder bores within said housing in commu nication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge ,stroke; a rotary drive shaft; a cam carried by said drive shaft forrecip'rocating said pistons; an openended cavity in each of said pistons, valve means comprising a disc means carried within each piston adjacent its open end arranged for limited axial movement and effective to open and close said cavity; a movable'memher carried by each'piston and adapted toengage said valverrneans to vary the position relative tothe cylinder at which said valve means closes said piston cavity,rsaid movable members thereby being effective to vary the eifective discharge stroke of said pistons and the displacement of said pump, a-source of fluid pressure, and an annular plate mounted for limited axial movement in response 'to the pressure value of said fluid pressure, said plate upon movement in one direction in response to an increase in the pressure value of Said fluid-pressure permitting said members to cause movement of said valve means relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke ;to :thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pumpdisplacement.

7. ha pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circularsen'es of cylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an open-ended cavity-in each of said pistons, poppet valve means comprising a valve proper and an axially extending stem portion carried by each piston and effective to open and close said cavity, means mounting said poppet valve means for movement relative to each piston, and means to limit the movement or said poppet valve means; a movable member carriedby eachpiston and adapted to engage said valve means to vary the position relative to the cylinder at which Said valve means closes said piston cavity, said movable members thereby being effective to vary the effective dischargestroke of said pistons and the displacement of said pump, a source of fluid pressure, and an annular plate mounted lforllimited axial movement in .responsetothe pressure value of said fluid pressure, said plate upon movement in one direction in response to an increase in the pressure value of said fluid pressure permitting said members to cause movement of said valve means relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pump displacement.

8. In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series of cylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an open-ended cavity in each of said pistons, disc valve means carried by each piston for limited axial movement and effective to open said cavity upon movement in one direction and close said cavity upon movement in another direction; a movable member carried by each piston and adapted to engage said valve means to vary the position relative to the cylinder at which said valve means closes said piston cavity, said movable members thereby being eifective to vary the effective discharge stroke of said pistons and the displacement of said pump, a source of fluid pressure, and an annular plate mounted for limited axial movement in response to the pressure value of said fluid pressure, said plate upon movement in one direction in response to an increase in the pressure value of said fluid pressure permitting said members to cause movement of said valve means relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pump displacement.

9. In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series of cylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an open-ended cavity in each of said pistons, valve means comprising a poppet having a valve proper adapted to seat on the open end of each piston and effective to open and close said cavity; a movable member carired by each piston and adapted to engage said valve means to vary the position relative to the cylinder at which said valve means closes said piston cavity, said movable members thereby being eifective to vary the effective discharge stroke of said pistons and the displacement of said pump, biasing means engaging said movable members and tending to urge said members to positions providing minimum pump displacement, a source of fluid pressure, a fluid motor carried by said housing and responsive to the pressure value of said fluid pressure, and an annular plate interconnected wtih said fluid motor and mounted for limited axial movement in response to the pressure value of said fluid pressure, said plate upon movement in one direction in response to an increase in the pressure value of said fluid pressure permitting said members to cause movement of said valve means relative to said piston whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pump displacement.

10. A pump as recited in claim 9 wherein said fluid motor in response to an increase in the pressure value of said fluid pressure above a predetermined pressure value causes movement of said plate in said one direction to thereby decrease pump displacement.

11. A pump as recited in claim 10 wherein spring means engages said plate and opposes movement of said plate in said one direction, said spring means being adapted to cause movement of said plate .in said another direction when said fluid motor is ineffective to thereby increase pump displacement.

12. In a pump of the type in which a plurality of cylinders are arranged in circular series with their cylinder axes parallel, the combination of a housing provided with an inlet and an outlet; means defining a circular series of cylinder bores within said housing in communication with said inlet and said outlet; a plurality of pistons, each received in a cylinder bore for reciprocation therein so as to having a suction stroke and a discharge stroke; a rotary drive shaft; a cam carried by said drive shaft for reciprocating said pistons; an openended cavity in each of said pistons, valve means comprising a disc means carried within each piston adjacent its open end arranged for limited axial movement and effective to open and close said cavity; a movable member carried by each piston and adapted to engage said valve means to vary the position relative to the cylinder at which said valve means closes said piston cavity, said movable members thereby being eifective to vary the eflective discharge stroke of said pistons and the displacement of said pump, biasing means engaging said movable members and tending to urge said members to positions providing minimum pump displacement, a source of fluid pressure, a fluid motor carried by said housing and responsive to the pressure value of said fluid pressure, and an annular plate interconnected with said fluid motor and mounted for limited axial movement in response to the pressure value of said fluid. pressure, said plate upon movement in one direction in response to an increase in the pressure value of said fluid pressure permitting said members to cause movement of said valve means relative to said pistons whereby said cavities remain open during a greater portion of the discharge stroke to thereby decrease pump displacement than upon movement of said plate in another direction in response to a decrease in the pressure value of said fluid pressure to thereby increase pump displacement.

13. A pump as recited in claim 12 wherein said fluid motor in response to an increase in the pressure value of said fluid pressure above a predetermined pressure value causes movement of said plate in said one direction to thereby decrease pump displacement.

14. A pump as recited in claim 13 wherein spring means engages said plate and opposes movement of said plate in said one direction, said spring means being adapted to cause movement of said plate in said another direction when said fluid motor is ineffective to thereby increase displacement.

References Cited in the file of this patent UNITED STATES PATENTS 2,459,303 Baker Jan. 18, 1949 

